1. Field of the Invention
The present invention relates to a thin film transistor device having a matrix circuit for applying an electric field to a liquid crystal display device and a driving circuit for driving the matrix circuit.
2. Description of the Related Art
An active matrix type liquid crystal display is known to the art. The conventional active matrix type liquid crystal display comprises a matrix circuit for applying an electric field to a liquid crystal display device and a peripheral driving circuit for driving the matrix circuit, each circuit being formed of field effect type thin film transistors.
FIG. 5 shows one example of the circuit structure of such a conventional active matrix type liquid crystal display.
This active matrix type liquid crystal display comprises a matrix circuit 51, one peripheral driving circuit 52 for supplying a scanning signal to an address bus of the matrix circuit 51, and another peripheral driving circuit 53 for supplying a display signal to a data bus of the matrix circuit 51. In the matrix circuit 51, there are formed a large number of scanning electrodes arranged in row and a large number of display electrodes 55 arranged in column, and a thin film transistor 57 for the matrix circuit 51 is formed for each pixel 56 (liquid crystal) corresponding to each cross point of the scanning electrodes 54 and the display electrodes 55. A gate electrode of each thin film transistor 57 is connected to each scanning electrode 54, and a source electrode is connected to the display electrode 55. A drain electrode of each thin film transistor 57 is connected to a pixel capacitive element 58 having a transparent electrode connected to each pixel in parallel. One peripheral circuit 52 comprises a thin film transistor (not shown) for said one peripheral circuit, which is connected to one end of the scanning electrode 54. The other peripheral circuit 53 comprises a thin film transistor for said other peripheral circuit, which is connected to one end of the scanning electrode 55. Then, if the thin film transistor 57 for the matrix circuit is turned on, the display data is written in the capacitive element 58 in the form of an electric charge. If the thin film transistor 57 for the matrix circuit is turned off, the pixel 56 is driven in a predetermined time by the written electric charge.
According to such a conventional thin film transistor device, the thin film transistor 57 forming the matrix circuit 51 and the thin film transistor forming the peripheral driving circuit were formed in the same structure. Therefore, characteristics of the transistor such as a switching speed of the thin film transistor, a cut-off speed and the like were the same in both transistors.
In recent years, display with extremely high precision has been required in the active matrix type liquid crystal display, and it has been needed that the number of thin film transistors, which form the matrix circuit 51 and the peripheral driving circuits 52 and 53, is increased.
However, as the number of thin film transistors is increased, the current consumption in the entire device is increased. In order to solve this problem, the cut-off current of each thin film transistor 57 must be controlled to be small. On the other hand, the switching speed of each thin film transistor must be increased in accordance with increase in the number of thin film transistors. However, as is well-known, the on-current must be increased in order to increase the switching speed, and the cut-off current is also increased by the increase in the on-current. For this reason, the above two requirements cannot be satisfied simultaneously.